Monocarbide precipitation-strengthened nickel base alloys and method for producing same

ABSTRACT

NICKEL BASE ALLOYS OF 15-22% CR,3-12% MO; AT LEAST ONE OF THE GROUP CONSISTING OF 6-9% TA, 3-5% CB AND 3-6% V, AND 0.03-0.15% C, BALANCE NICKEL AND RESIDUAL IMPURITIES ARE STRENGTHENED BY PRECIPITATING DISPERSED MONOCARBIDES HAVING PARTICLE SIZES OF LESS THAN 250 ANGSTROMS BY SEQUENTIALLY SOLUTION HEAT TREATING THE ALLOY AT A TEMPERATURE OF 2250-2300*F. QUENCHING AND COLDWORKING IN REPETIVE CYCLES. THE MATERIAL IS THEREAFTER FINALLY ANNEALED, QUENCHED AND AGED TO PRODUCE THE MONOCARBIDE PRECIPITATE.

March 14, 1972 P. s. KOTVAL 3,649,378 MONOCARBIDE PRECIPITATION-STRENGTHENED NICKEL BASE ALLOYS AND METHOD FOR PRODUCING SAME Flled June 20, 1969 2 Sheets-Sheet 1 FINE PART/CL ES 0F MC CARE/0E DISTRIBUTED (Hv Assoc/A TION WITH PLANAR LA TT/cL' DEFEC Ts) THROUGHOUT THE ALLOY MA TR/x.

GRAIN BOU/vOARY W/TFI' M23 a CARBIDE PRIMARY MC CARE/DE PARTICLE PAR TIALL) DISSOL VED OUR/N6 HIGH TE MPE RA TURE SOLUTION HE A T TRE A THE N T (APPROX. 70, 000 x) GRAIN BOUNDARY M23 0 CARBIDE.

/$TRENG7'HN/NG MC CA RB/DE' F/HELY D/SPERSED.

PARTICLES OF MC CARBIDE.

SCHEHAT/C REPRESENTATION OF SURFACE OF THE STRENGTHENED ALLOY.

(APPROX. aooox) INVENTOR I Pesho fan Sohrab Korval ATTORN EY March 14, 1972 p 5 KOTVAL 3,649,378

MONOCARBIDE PRECIPITATION-STRENGTHENED NICKEL BASE ALLOYS AND METHOD FOR PRODUCING SAME Filed June 20, 1969 2 Sheets-Sheet 2 AS CAST INGOT OR BILLET SOLUTION HEAT TREATMENT AT 2250-2300") T0 PART/ALLY DISSOLVE THE "PR/MARY "Mc CARE/0E5 AND TO HOMOGEN/ZE OUEIVCH &

COLD ROLL HOMOGENIZE AT HIGH SOLUTION HEA 7 mm TMENT (2250-2300F) TEMPERATURE OUENCH COLD ROLL SOLUTION ANNEAL AT H/GH SOLUTION HEAT mm mam (2250 -2aooF) TEMPERATURE QUENCH AGE AT TEMPERATURE IN RANGE OF Il00-IJ50F MC CARE/0E PREC/P/ TA TION $TRENGTHENED AL LOV INVENTOR Peshoran Sohrab Kotval United States Patent 3,649,378 MONOCARBIDE PRECIPITATION-STRENGTH- ENED NICKEL BASE ALLOYS AND METHOD FOR PRODUCING SAME Peshotan Sohrab Kotval, Indianapolis, Ind., assignor to Cabot Corporation, Boston, Mass. No Drawing. Filed June 20, 1969, Ser. No. 835,201 Int. Cl. C22c 19/00; C21d 7/00 U.S. Cl. 14812.7 7 Claims ABSTRACT OF THE DISCLOSURE Nickel base alloys of 15-22% Cr, 3l2% M; at least one of the group consisting of 69% Ta, 3-5 Cb and 36% V, and 0.03-0.15 C, balance nickel and residual impurities are strengthened by precipitating dispersed monocarbides having particle sizes of less than 250 angstroms by sequentially solution heat treating the alloy at a temperature of 22502300 F, quenching and coldworking in repetitive cycles. The material is thereafter finally annealed, quenched and aged to produce the monocarbide precipitate.

This invention relates to precipitation strengthened nickel base alloys and more particularly to alloys of this type which are strengthened by monocarbides. The invention also relates to a method for producing monocarbide precipitation strengthened nickel base alloys.

Prior art wrought nickel base alloys are strengthened by two principal mechanisms: (a) solution strengthening which has also been described as matrix stiffening and (b) precipitation strengthening.

In solution strengthening, a solute element, usually of large atomic size, strengthens the nickel-chromium matrix by its being in solid solution therewith. Molybdenum and tungsten solute atoms typically are used to produce this eifect in nickel base alloys. This type of strengthening is seriously limited by the natural limits of solubility of the strengthening atom in the nickel base matrix.

In precipitation strengthening, the solid solution matrix is usually super-saturated with respect to two or more solute elements and therefore, upon appropriate heat treatments, an aging reaction can occur causing the precipitation of phases (which are combinations of solute elements) which strengthen the alloy. In general, a strengthening precipitate should have coherency with the matrix, be well distributed within the body of the grain and be reasonably thermally stable. Typical examples of precipitation strengthening elements are aluminum and titanium which alone or in combination with other elements such as columbium and tantalum result in the formation of precipitation strengthening phases.

Both solution strengthened and precipitation strengthened nickel base alloys contain suflicient carbon so as to form monocarbides of a simple face centered cubic structure whenever monocarbide forming elements such as Ti, Ta, Cb, V, Zr and Hf are present. 'It follows that since one or more of these elements are usually present in precipitation strengthened alloys, and are sometimes present in solution strengthened alloys, that the monocarbides will be present. It is important to note that these monocarbides are usually in the form of undissolved primary particles present in the alloy matrix but which do not effectively strengthen the same. Typically, primary monocarbide particles have particle diameters of between 10 and 20 microns in nickel base alloys.

It has been observed that when a temperature of 2370 F. is maintained during the austenitization of certain steels, the monocarbides present in such steels can be dissolved and subsequently re-precipitated during aging 3,649,378 Patented Mar. 14, 1972 in the range 1000 F.-1300 F. Investigators have noted that said re-precipitation occurs in association with lattice defects in the matrix. However, prior art attempts to dissolve the primary monocarbides in nickel base alloys by raising the solution heat treatment temperature to a point high enough to cause dissolution of the primary monocarbides have been completely unsuccessful because of the inherently low melting point of most prior art compositions.

Another fundamental reason preventing the direct application of the stainless steel results to nickel base alloys is that the requisite precipitate morphology needs as a precondition certain characteristics of the matrix (is. certain ranges of stacking fault energy and accompanying dislocation reaction possibilities) which are not present in most nickel base alloys.

In solution-strengthened nickel-base alloys, the amounts of monocarbide-forming elements are usually at low enough levels so as to be in solid-solution in the alloy matrix after a portion of the monocarbide-forming element has combined with the available carbon present in the alloy to form the monocarbide.

The main object of the invention is to provide a nickel base alloy composition and method of making same wherein the heretofore latent strengthening characteristics of the monocarbides are utilized in such manner that the monocarbides serve as precipitation strengthening phases.

Another object of the invention is to utilize nickel base alloy compositions having monocarbide-forming elements at levels which heretofore were such as to permit only solid solution strengthening, as precipitation strengthening compositions.

A more specific object is to provide a nickel base alloy having dispersed therein primary monocarbides having less than 5 microns particle diameter and also having precipitated monocarbide particles of less than 250 angstroms 1n size.

These and still other objects will suggest themselves to those skilled in the art from the following disclosure, considered in conjunction with the following drawings wherein:

FIGS. 1 and 2 are schematic representations of the microstructure of the alloys of the present invention; and FIG. 3 is a diagrammatic process flow chart outlining the steps required for making the alloys of the invention.

According to the invention, nickel-base alloy compositions are provided having a new microstructure. This microstructure is comprised of primary monocarbides having less than 5 micron particles size in addition to uniformly dispersed precipitated monocarbide particles of less than 250 angstroms particles size. These alloy compositions are precipitation-strengthened by uniformly dispersed monocarbides and consist essentially by weight of 1522% Cr, 312% M0, at least one member selected from the group consisting of 6-9% Ta, 35% Cb and 36% V; the total content of Ta, Cb and V not exceeding 9%, and 0.030.l5% C., the balance being nickel and residual impurities. The impurities should consist of not more than 7% Fe and not more than 8% Co.

The ability to produce the abovementioned nickel base alloy results from the discovery that by controlling the level of certain impurties in the nickel base alloy, (i.e. Fe) it is possible to solution heat treat the alloy at temperatures of about 2280 F. without causing liquation and melting. Although, ideally, higher temperatures would normally be required to dissolve monocarbides, it is possible to achieve partial dissolution thereof by progressively subjecting the material to an alternating sequence of high temperature solution heat treatments of about 2280 F. and quenching together with controlled cold working steps.

Chromium is required in the alloy within the range disclosed above to provide strengthening and corrosion resistance. Less than Cr yields an alloy with minimal corrosion resistance; over 22% Cr yields an alloy with reduced ductility.

Molybdenum is present in the alloy within the ranges shown above to provide further solution strengthening and corrosion resistance as required. Molybdenum is preferred in the alloy, although tungsten may replace molybdenum in whole or in part. Tungsten may be present in greater quantities than molybdenum, i.e. up to a maximum of 16%, but preferably about 12% nominally. Carbon must be present in the alloy within the range of about 0.03 to 0.15% by weight to promote the formation of carbides in the alloy. Less than 0.03% carbon is insufiicient to produce the carbides while over 0.15% carbon tends to yield a more brittle alloy. Alloys containing over 0.15% carbon are more difficult to work.

The alloy system of this invention must contain at least one of the group including tantalum, columbium, and vanadium. At least one of these elements must be present in the alloy together with carbon to provide the metal monocarbides that are precipitated in the nickelchromium (molybdenum) matrix. The presence of these precipitated carbides together with the critical processing steps that promote the controlled precipitation are the heart of the present invention. Each element in this group must be present within the ranges stated above when that element is the principal carbide former. In some cases it may be desirable to substitute Hf, Zr or Ti for the element selected from the group consisting of Ta, Cb and V. The total content of elements in these groups must not exceed 9%. Iron may be present up to a maximum of 7%. This requirement results from the fact that the higher iron content tends to lower the melting point of the alloy thereby prohibiting the higher solution heat treatment temperature required to cause the dissolution of the primary monocarbides so that subsequent reprecipitation can be achieved. Cobalt may be present up to a maximum of 8%. Higher cobalt contents tend to lower the matrix stacking fault energy and thereby cause impediments in the nucleation of the dislocation reaction which is a necessary precondition for the desired morphology of precipitation.

Boron, silicon, maganese, magnesium, and copper up to a total of about 2.5% may be present in the alloy within the ranges known in the art to be effective to enhance certain characteristics associated with these elements; i.e. the deoxidation step, casting fluidity, ductility and the like.

The balance of the alloy is nickel and adventitious impurities generally known to be present in this class of alloys.

Referring now to the drawings, FIG. 1 is a threedimensional representation of the microstructure as would be observed in a thin foil sample using transmission electron microscopy. FIG. 2, on the other hand, is a representation of microstructure as would be observed by a replica of the surface of the specimen. It should be understood that this type of structure cannot be observed and resolved by normal optical metallographic techniques. In FIGS. 1 and 2, the microstructure of the alloys of the invention as shown, consists of the usual grain structure with some precipitation of the chromium rich M C carbide phase at the grain boundaries. Unique to this microstructure however is the highly refined particles of the primary MC monocarbides P. The actual size of the primary MC monocarbides P shown is about 12 microns. The unique structure further has dispersed within it, fine precipitates of MC monocarbides K which are associated with sheets of planar lattice defects. The precipitates K are of about 250 angstroms or less in size and being coherent with the matrix substantially strengthen it.

Referring now to FIG. 3, the alloys of the invention can be made by providing an alloy material with 1522% Cr, 312% M0, at least one member selected from the group consisting of 69% Ta, 3-5% Cb and 36% V, the total of Ta+Cb+V not exceeding 9%, and 0.03- 0.15 C, the balance being nickel and residual impurities, said impurities consisting of not more than 7% Fe and not more than 8% Co, said alloy being in a form capable of being cold-worked.

The material is solution heat-treated at a temperature within the range of 2250-2300 F. for a period of about 24 hours to at least partially dissolve the primary" monocarbide particles and thereby causing the monocarbideforming element and the carbon to be put into solid solution in the alloy matrix. Following this solution-heattreatment the material is water-quenched and cold worked. Until the final desired dimensions of the product are achieved, the above mentioned sequence of solution-heattreatment followed by quenching and cold-working is repeated. Thereafter, the cold-worked material is annealed for a period not exceeding one hour within the solution heat-treatment temperature range of 2250-2300 F. After annealing, the material is water-quenched to about ambient temperature and is thereafter aged at a temperature within the range of 1100 F.1350 F. for a period of 24 to hours whereby a sufiicient volume fraction of the strength-giving precipitated fine monocarbides will occur.

It should be understood that the present invention also contemplates that in certain situations it may be desirable to substitute a member selected from the group consisting of. Hf, Zr and Ti for the monocarbide-forrning elements previously discussed, i.e. Cb, Ta and V.

The invention will now be illustrated by the following examples:

EXAMPLE I A 5 1b. heat of material consisting of, by weight, 21.9% Cr, 8.83% M0, .067% C, 3.86% Cb, 4.9% Fe, the balance being nickel; was electron-beam melted and cast into a 1.5 round bar. This bar was solution heat treated at a temperature of 2282 F. for a period of 24 hours and water-quenched. Thereafter, the material was sequentially cold-worked, solution-heat-treated at 2282 F. and quenched. After the process was repeated five times, the resultant 0.025 sheet produced was solution heat treated (annealed) for a period of 1 hour at 2282" F. and quenched. Thereafter, the material was cut into various specimens, each of which was aged at a diiferent temperature and time. The range of aging temperatures was 1100 F.1350 F. and the range of aging times was 0.5 hour to 1500 hours. In all cases, thin foil transmission electron microscopy revealed that the requisite structure, as depicted in FIGS. 1 and 2, was established in the alloys.

Several 0.025" thick sheet specimens of the material of Example I, were aged for 48 hours at 1202 F. following solution heat treatment and quenching as described above.

The following tensile properties were observed. At room temperature the average yield 0.2% offset) stress for the aged material was 76,000 p.s.i. This represents an increase of about 100% from the average yield (0.2% offset) stress of the as-solution heat-treated and quenched material which at room temperature was 38,000 p.s.i. At 1300 F., the average yield (0.2% offset) stress of the aged material was 46,000 p.s.i. as compared to an average yield (0.2% oifset) stress of 34,000 p.s.i. for the as-solution heat treated and quenched material at 1300" F. Ductility values for all these specimens were found to be in the range of 15-20% elongation.

EXAMPLE II A 5 1b. heat of material consisting of (by weight) 20.34% Cr, 8.90% Mo, 0.11% C, 8.58% Ta and 4.80% Fe, the balance being nickel was melted and processed in the same manner as described in connection with Example I. The material was thereafter structurally analyzed by electron microscopy which verified the formation of pre* cipitated monocarbides as described in Example 1.-

EXAMPLE III A 5 lb. heat of material consisting of (by weight) 22.2% Cr, 9.28% Mo, 0.04% C, 3.93% V and 4.75% Fe, the balance being nickel was melted and processed in the same manner as described in Example I. Analysis by electron microscopy verified the formation of precipitated monocarbides as described in Example 1.

While the invention has been described in connection with specific alloy compositions and heat treatment steps, it should be understood that minor variations may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A precipitation-strengthened nickel-base alloy strengthened by the precipitation throughout the matrix thereof of fine particles of the monocarbides of elements from the group consisting of Ta, Cb and V, and having the following composition by weight: to 22% Cr, 3 to 12% M0, at least one of the group consisting of 6 to 9% Ta, 3 to 5% Cb and 3 to 6% V provided that the total of Ta, Cb and V does not exceed about 9%, and 0.03 to 0.15% C., the balance of said alloy being nickel and residual impurities provided that the Fe content does not exceed 7% and the Co content does not exceed 8%, said monocarbides being dispersed therein both as primary particles of up to about 5 microns in size and as finely precipitated particles of less than 250 angstroms in size.

2. A precipitation-strengthened nickel-base alloy as defined in claim 1 wherein said finely precipitated monocarbide particles occur mainly in association with planar lattice defects in the alloy matrix.

3. A precipitation-strengthened nickel-base alloy as defined by claim 1 wherein W is substituted for at least part of the Mo content.

4. -A precipitation-strengthened nickel-base alloy as defined by claim 1 wherein Zr, Ti or Hf is used in place of at least part of the content of elements from the group consisting of Ta, Cb and V.

5. A method of producing a precipitation-strengthend nickel-base alloy comprising the steps of:

(a) providing in a form suitable for cold working an alloy material consisting by weight of 15 to 22% Cr, 3 to 12% M0, at least one member from the group consisting of 6 to 9% Ta, 3 to 5% Cb and 3 to 6% V provided that the total of Cb, Ta and V does not exceed about 9%, and 0.03 to 0.15% C with the balance being nickel and residual impurities, provided that the Fe content does not exceed 7% and the Co content does not exceed 8%;

(b) solution heat treating said material at a temperature between about 2250 and 2300 F. to dissolve at least partially primary monocarbide particles present therein; thereafter (c) quenching the material; thereafter ((1) coldworking the material until the desired form is attained; thereafter (e) annealing the material at a temperature in the range of about 2250 to 2300 F. for a period not exceeding 1 hour; thereafter (f) quenching the material and thereafter (g) aging the material at a temperature of about 1100 to 1350 F. for a period of about 24 to 100 hours, thereby precipitating throughout the alloy matrix fine monocarbide particles of less than 250 angstroms in size.

6. A method of producing a precipitation-strengthened nickel-base alloy as claimed in claim 5 wherein steps (d), (e) and (f) are repeated as necessary in order to attain the desired form before aging step (g) is carried out.

7. A method as defined in claim 5 wherein W is used in place of at least part of the Mo content and Zr, Ti or HE is used in place of at least part of the content of elements from the group consisting of 6 to 9% Ta, 3 to 5% Cb and 3 to 6% V specified in step (a).

References Cited UNITED STATES PATENTS 3,046,108 7/ 1962 Eiselstein -171 3,069,258 12/1962 Haynes 75l71 3,085,005 4/ 1963' Michael et a1. 75-171 3,151,981 10/1964 Smith et al 75--171 3,372,068 3/1968 White 148---32.5 X 3,466,171 9/ 1969 Fletcher et al. 75171 3,497,349 2/1970 Eppich 75171 3,411,899 11/1968 Richards et al 75-171 CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 

